A recurrent problem in radiological practice is the adjustment of two images to compare them, or to measure, calculate and visualize their difference in marked regions.
Typical radiological examinations are those of the vertebrae of the spine on radiographic images, or images of the stomach and blood vessels.
The differences introduced by patient positions, biomechanical motions such as flexion and extension, or exposure viewpoint make simple superimposition of two images not straightforward. In the prior film-based art, superposition of two film images is achieved by superimposing one film onto another, whilst at the same time sliding the second film relative to the first one by successive translations and rotations such that the corresponding anatomic structures of interest geometrically match to each other. This operation is typically performed when both superimposed images are displayed on a lightbox, or alternatively positioned in front of a light intensive lamp. The resulting image that the radiologist examines is a combination of densities registered on both films in the regions where both films overlap, and equals the original film density in all other areas where only a single sheet is present.
The drawbacks of the film-based prior art can be summarized as follows.
It is difficult to correctly match by visual inspection structures that mutually correspond.
The act of superimposing the images must be repeated every time the images are to be matched, or when a sequence of structures of interest has to be matched. The manual procedure is thus fatiguing and error-prone.
Additional operations on the superimposed film combination, such as geometric measurements (distances, angles), are difficult to effectuate.
The film-based superimposition cannot be archived for further referral.
Different structures recorded on a single film cannot be registered with one another.
It is an aspect of this invention to provide a method to perform measurements on digital images that overcomes the drawbacks of the prior art.
Other aspects of the present invention will become apparent from the description given below.